The steady evolution of mobile telecommunication brings about changes to the Radio Access Network (RAN) and/or the Core Network (CN) of a telecommunications network. Such changes are preferably implemented in a step-by-step migration. As an example, a network operator providing service coverage according to the Universal Mobile Telecommunication System (UMTS) may want to move voice services to Voice over Internet Protocol (VoIP) using an Internet Protocol Multimedia Subsystem (IMS) in the CN in conjunction with a RAT according to Long Term Evolution (LTE). Since UMTS provides voice services in the circuit-switched domain, the operator would have to provide ubiquitous LTE coverage at the first day of VoIP service, in order to prevent that a call initiated in the packet-switched domain according to LTE is discontinued as the mobile caller leaves the LTE coverage area.
Single Radio Voice Call Continuity (SRVCC) according to standard document 3GPP TS 23.216 (Release 11, Version 11.7.0) specifies a technique for service continuation of IMS voice calls. The IMS voice call can, for example, originate in the packet-switched domain using a conversational VoIP bearer according to the Quality of Service (QoS) Class Identifier “QCI1”. The QCI1 VoIP bearer is merely one example for an Evolved Packet System (EPS) bearer. In UMTS and GPRS, data sessions are established differently using the Packet Data Protocol (PDP) context procedure. SRVCC provides the ability of transferring a voice call from the packet-switched domain of LTE using VoIP and IMS to the circuit-switched domain of UMTS. SRVCC thus provides a fallback position when introducing VoIP services according to the LTE standard.
Standard documents 3GPP TS 23.237 (Release 12, Version 12.1.0) and 3GPP TS 24.237 (Release 11, Version 11.4.0) specify the SRVCC transfer for video and voice calls in an Alerting Call State. The Alerting Call State of a session may be defined as the state of a party engaged in a session that is in an early dialog state according to Sect. 12.1 of RFC 3261. Alternatively or in addition, the Alerting Call State of the call may be defined as the state of a session with a speech media component after exchanging an initial “SIP INVITE” request that initiates the dialog and a “SIP 180” Ringing response that indicates the alerting, and before exchanging a SIP final response for the initial “SIP INVITE” request that establishes the session according to Clause 12.1 of standard document 3GPP TS 24.237.
Due to a potential radio interruption during the SRVCC transfer, the state of the calling User Equipment (UE) and the state of one or more components of the telecommunications network, such as a Mobile Switching Center (MSC) server, may become out of synchronization. To tackle the possible incompatibility of call states between the calling UE and the MSC server, the standard document 3GPP TS 24.237 recommends that the MSC server verify the call state using the “STATUS ENQUIRY” procedure of standard document 3GPP TS 24.008 (Release 12, Version 12.0.0). If the call states are incompatible, the transferred session is released according to standard document 3GPP TS 24.237, Clause 12.6.3, Note 3.
Document WO 2011/139045 A2 teaches a technique for synchronizing different call states at the remote other end of the dialog. Here, an incompatibility is caused by the called UE answering the call just before the called UE is handed over from the packet-switched mode to the circuit-switched mode according to SRVCC. No solution is provided for synchronizing the calling UE and the telecommunications network from which the call is originating.
Thus, incompatible session states can arise in the prior art and lead to an unexpected termination of the session.